feat(python): add public take_offsets method on Permutation (#3375)

Closes #3243.

This PR exposes a new public api `Permutation.take_offsets(offsets:
list[int])`, since users initially had to call __getitems__ directly to
batch-fetch rows by position.

Currently, the name matches the existing `Table.take_offsets` pattern,
and now the dunder `__getitem__` and `__getitems__` now delegate to it.

Also, fixes a parse error when `PermutationReader::take_offsets` gets an
empty list. Now returns an empty `RecordBatch` with the correct schema
instead. Bundled this because without the fix the new public API blows
up on a perfectly reasonable input.

`__getitems__` is preserved since PyTorch's batched DataLoader requires
it.

### Testing

- Added 3 new Rust tests for empty offsets including permutation table
with Select::All, Select::Columns, and identity path
- Added 3 new Python tests for the public API including a happy case,
and empty input on both identity and permutation

clippy, format, check all clean!

cc: @westonpace

rust/lancedb/src/dataloader/permutation/reader.rs

@@ -450,6 +450,10 @@ impl PermutationReader {
     }
 
     pub async fn take_offsets(&self, offsets: &[u64], selection: Select) -> Result<RecordBatch> {
+        if offsets.is_empty() {
+            return Ok(RecordBatch::new_empty(self.output_schema(selection).await?));
+        }
+
         if let Some(permutation_table) = &self.permutation_table {
             let offset_map = self.get_offset_map(permutation_table).await?;
             let row_ids = offsets
@@ -955,4 +959,62 @@ mod tests {
             .to_vec();
         assert_eq!(idx_values, &all_idx_values[4997..5000]);
     }
+
+    #[tokio::test]
+    async fn test_take_offsets_empty_identity_reader() {
+        let base_table = lance_datagen::gen_batch()
+            .col("idx", lance_datagen::array::step::<Int32Type>())
+            .into_mem_table("tbl", RowCount::from(10), BatchCount::from(1))
+            .await;
+
+        let reader = PermutationReader::identity(base_table.base_table().clone()).await;
+
+        let batch = reader.take_offsets(&[], Select::All).await.unwrap();
+
+        assert_eq!(batch.num_rows(), 0);
+        assert_eq!(batch.num_columns(), 1);
+        assert_eq!(batch.schema().field(0).name(), "idx");
+    }
+
+    #[tokio::test]
+    async fn test_take_offsets_empty_with_permutation_table() {
+        let (base_table, row_ids_table, _) = setup_permutation_tables(5).await;
+
+        let reader = PermutationReader::try_from_tables(
+            base_table.base_table().clone(),
+            row_ids_table.base_table().clone(),
+            0,
+        )
+        .await
+        .unwrap();
+
+        let batch = reader.take_offsets(&[], Select::All).await.unwrap();
+
+        assert_eq!(batch.num_rows(), 0);
+        assert_eq!(batch.schema().fields().len(), 2);
+        assert_eq!(batch.schema().field(0).name(), "idx");
+        assert_eq!(batch.schema().field(1).name(), "other_col");
+    }
+
+    #[tokio::test]
+    async fn test_take_offsets_empty_with_column_selection() {
+        let (base_table, row_ids_table, _) = setup_permutation_tables(5).await;
+
+        let reader = PermutationReader::try_from_tables(
+            base_table.base_table().clone(),
+            row_ids_table.base_table().clone(),
+            0,
+        )
+        .await
+        .unwrap();
+
+        let batch = reader
+            .take_offsets(&[], Select::Columns(vec!["idx".to_string()]))
+            .await
+            .unwrap();
+
+        assert_eq!(batch.num_rows(), 0);
+        assert_eq!(batch.num_columns(), 1);
+        assert_eq!(batch.schema().field(0).name(), "idx");
+    }
 }